1. Field of the Invention
The present invention relates generally to the field of mode discrimination means in laser cavities, and in particular, mode discrimination in macroscopic cavities wherein a vast number of modes may otherwise be sustained.
2. Description of the Related Art
The present invention relates generally to the field of lasers and optical resonator design, and in particular, to the fields of disk and spherical lasers. Also, the invention relates to cavity structure designs that utilize multi-layer dielectric (MLD) thin film reflectors that provide a high degree of mode selection. Inventive matter disclosed herein is related to co-pending U.S. patent application Ser. No. 09/839,254, filed Apr. 20, 2001.
Laser cavities of the disk and spherical geometries have become an increasingly intensive field of research; in particular, for such lasers that are fabricated on a miniature or microscopic scale. In the latter case, the predominant means of cavity reflection is through total internal reflection (TIR), which provides an extremely high cavity Q. Such reflective means normally manifest in “whispering modes,” which propagate at angles below the critical angle for TIR. These microdisk and microsphere lasers are very effective in cases involving evanescent coupling to an adjacent dielectric structure; however, they are known to contain a very large number of competing high-order modes. In addition, the coupling of these whispering modes for useful work is difficult for applications not utilizing evanescent coupling.
In recent years, theoretical studies have been performed on the development of derivation methods for cylindrical and spherical multilayer structures, which are aimed at providing an accurate description of the reflection coefficients and modal characteristics of these cavities. These studies address circular confinement structures with cavity dimensions on the order of the wavelengths studied. However, none of these studies are found to address the issues of applying similar circular Bragg reflectors for larger cavities of the scale used for gas and larger solid state cavities. Furthermore, these previous studies also entertain only the use of conventional MLD filters, with a large real refractive index difference, nH−nL=Δn>1, for the layer pairs, and with an accordingly small number of layers required for high reflection.
The use of interference structures to enable high spectral resolving power in reflecting coatings has been described by Emmett (U.S. Pat. No. 4,925,259), wherein a very large number of alternating dielectric layers possessing a very small difference in refractive indices is used for application in high power flashlamps. The described coatings are utilized primarily for providing a high damage threshold to the high irradiance experienced in the flashlamp enclosure, as well as for obtaining a well-resolved pump wavelength for use in the described flashlamp.
The control of transverse modes in semiconductor lasers, primarily VCSEL's, has been reported by several research groups in the last decade. These latter reports utilize a circular Bragg grating structure as a complement to the planar Bragg mirrors of a conventional, high Q semiconductor cavity. Such circular Bragg gratings do not form the initial resonant cavity, but rather, aid in controlling relatively low Q, transverse modes of an existing Fabry-Perot structure. In such cases, the resultant control of transverse propagation may allow lowered thresholds, or enhanced stability.
Earlier, large-scale, laser designs of a circular geometry operated on very different principles than the microlasers, utilizing primarily gas laser mediums and metallic reflectors. In these earlier designs, optical power could be coupled for useful work at the center of the cavity, such as for isotope separation, or by using a conical reflector. Since, in these latter cases, laser modes that concentrated energy at the cavity's center were needed, some means for blocking the whispering-type modes was generally required. Such mode suppression was usually accomplished through radial stops; however, these stops only provided the most rudimentary mode control, in addition to hampering the efficient operation of the laser. Because of such issues, disk and spherical lasers have not supplanted standard linear lasers for any applications requiring substantial optical power or a high degree of mode selection.